UTF-8 encoding is assumed

I'm creating a dancer2 request with some Shift-JIS-encoded text in it. These are valid octets, suitable for transmission. When the body is parsed a warning is emitted, because it's not valid UTF-8 -- this shouldn't be assumed.

stack trace:

Invalid UTF-8 in body parameters; leaving bytes unchanged at /Volumes/chambord/Users/ether/.perlbrew/libs/43.10@std/lib/perl5/Dancer2/Core/Request.pm line 481.
        Dancer2::Core::Request::_invalid_utf8(Dancer2::Core::Request=HASH(0x7afa1d018), "\x{82}\x{e2}\x{82}\x{c1}\x{82}\x{bd}", "body parameters") called at /Volumes/chambord/Users/ether/.perlbrew/libs/43.10@std/lib/perl5/Dancer2/Core/Request.pm line 463
        Dancer2::Core::Request::_decode_bytes(Dancer2::Core::Request=HASH(0x7afa1d018), "\x{82}\x{e2}\x{82}\x{c1}\x{82}\x{bd}", "body parameters") called at /Volumes/chambord/Users/ether/.perlbrew/libs/43.10@std/lib/perl5/Dancer2/Core/Request.pm line 438
        Dancer2::Core::Request::_decode(Dancer2::Core::Request=HASH(0x7afa1d018), "\x{82}\x{e2}\x{82}\x{c1}\x{82}\x{bd}", "body parameters") called at /Volumes/chambord/Users/ether/.perlbrew/libs/43.10@std/lib/perl5/Dancer2/Core/Request.pm line 446
        Dancer2::Core::Request::_decode(Dancer2::Core::Request=HASH(0x7afa1d018), HASH(0x7afa67240), "body parameters") called at /Volumes/chambord/Users/ether/.perlbrew/libs/43.10@std/lib/perl5/Dancer2/Core/Request.pm line 441
        Dancer2::Core::Request::_decode(Dancer2::Core::Request=HASH(0x7afa1d018), Hash::MultiValue=HASH(0x7af8d6990), "body parameters") called at /Volumes/chambord/Users/ether/.perlbrew/libs/43.10@std/lib/perl5/Dancer2/Core/Request.pm line 402
        Dancer2::Core::Request::body_parameters(Dancer2::Core::Request=HASH(0x7afa1d018)) called at /Volumes/chambord/Users/ether/.perlbrew/libs/43.10@std/lib/perl5/Dancer2/Core/Request.pm line 128
        Dancer2::Core::Request::_body_params(Dancer2::Core::Request=HASH(0x7afa1d018)) called at /Volumes/chambord/Users/ether/.perlbrew/libs/43.10@std/lib/perl5/Dancer2/Core/Request.pm line 584
        Dancer2::Core::Request::_build_uploads(Dancer2::Core::Request=HASH(0x7afa1d018)) called at /Volumes/chambord/Users/ether/.perlbrew/libs/43.10@std/lib/perl5/Dancer2/Core/Request.pm line 92
        Dancer2::Core::Request::new("Dancer2::Core::Request", "env", HASH(0x7afed3ea0)) called at t/lib/Helper.pm line 164
...

By the way, this is bad code:

437:        if ( !is_ref($h) && !utf8::is_utf8($h) ) {

as is this:

458:    # If PSGI already gave us characters, avoid re-decoding.
459:    return $bytes if utf8::is_utf8($bytes);

You simply can't use is_utf8 that way. It does NOT convey whether a string is unicode characters vs utf8 bytes. (see perldoc utf8 for references.)

Start in Dancer2/Core/Request.pm around lines 402-481, following _decode, _decode_bytes, and _invalid_utf8. Read perldoc utf8 and reproduce the request with the Shift-JIS octets described in the issue. Done means request body parsing no longer assumes every incoming byte string is UTF-8 or emits the reported warning for valid non-UTF-8 input.